[doc] Use consistent spelling/capitalization of names

Consistently use the following spelling for document formats,
programming languages, and names:

- Markdown
- Hjson
- JSON
- Verilog
- HTML
- GitHub
- CSS
- Python

Also avoid abbreviations:
- repo -> repository

Signed-off-by: Philipp Wagner <phw@lowrisc.org>
diff --git a/util/reggen/gen_cheader.py b/util/reggen/gen_cheader.py
index 7f8836d..6d242cb 100644
--- a/util/reggen/gen_cheader.py
+++ b/util/reggen/gen_cheader.py
@@ -2,7 +2,7 @@
 # Licensed under the Apache License, Version 2.0, see LICENSE for details.
 # SPDX-License-Identifier: Apache-2.0
 """
-Generate C header from validated register json tree
+Generate C header from validated register JSON tree
 """
 
 import io